Field Report: Wheatstone Bridge

June 1, 2007

As part of the facility installation at Entercom Kansas City (profiled in the April issue of Radio magazine), we installed a Wheatstone Bridge router with G-series control surfaces. We knew that we wanted to build the facility around a router for maximum flexibility with our nine stations. These needs were met with the Bridge.

The system is built around card-configurable frames that can sample any combination of analog or digital audio sources and interface with logic control sources. Two frames can be connected to share as many as 64 sources and destinations using Wheatnet Audio Network cards and CAT-5e cable. As many as 48 frames can be connected using the Wheatnet Hub for a flexible and scalable building-wide system.

It wouldn't surprise me if the North Carolina-based company took inspiration from the hundreds of spans, viaducts and turntable drawbridges that surround the factory. The Bridge interconnects a family of audio interfaces and control devices into a cohesive and flexible system. Like islands, studios are no longer isolated or limited by cabling or router size.

Creating connections, processing audio and making future changes is easy when every audio source, destination or logic control resides in the same digital system. In our installation, Generation-series mixing surfaces take the place of the traditional console, and fader and switch commands are transmitted to their respective Bridge frame. Because no audio passes through the surface, all audio conditioning and mixing takes place within the digital confines of the frame.

Performance at a glance
Eight-channel microphone pre-amp boards
Eight stereo channel analog input and output boards
Eight and 16 channel digital AES input and output boards
Generation Series consoles in a variety of fader and output bus sizes
TCP/IP-based remote control panels
Extensive configuration, monitoring and control with XPoint GUI software
Direct TCP/IP communication with many popular audio automation systems
Wheatnet Hub 48-port centralized network controller

Control logic such as remote microphone on, off, cough and talk back functions are digitized and routed just like its audio counterpart. Recently, Wheatstone developed a TCP/IP-based logic control so stand-alone, multi-button panels and computer automation systems can easily communicate to the Bridge over Ethernet.

At the heart of this system are two classes of interface processors: the Bridge, capable of processing 512 analog or digital audio sources, and its scaled-down cousin the Satellite. The Bridge frame is suited for centralized operations where a high volume of signal processing is concentrated in one or more rooms. The Satellite frame is a cost-effective interface for smaller studio settings with fewer audio sources. Both devices have essentially the same function, but have different audio signal-handling capacities.


Each frame is configured with a CPU card and can be populated with a combination of mic pre-amp, line level analog, digital AES, logic and DSP mixing cards. Physical connections are made with 25-pin D-style, RJ-45 or BNC-type connectors.

The Microphone Card accepts eight mono microphone level (-50dBm) sources and provides eight mono direct outputs. The pre-amplified microphone signal is electronically balanced and introduced to a 24-bit A/D sampler set to the master clock rate. All eight digitized audio signals are inserted into a single Time Domain Multiplexing (TDM) channel on the router bus. The card also supplies 32V phantom power.

The Analog Eight Channel Stereo (16 mono) Input Card is a direct coupled, balanced input with a nominal level of +4dBu. Signals are buffered and digitized using 24-bit A/D converters. The Analog Eight Channel Stereo (16 Mono) Output Card can be configured in mono, stereo or 5.1 output modes. Outputs are direct coupled, balanced at 50Ω with a nominal level of +4dBu.

The Digital Eight or 16 Channel Input Card conforms to the AES-3 standard interface of +5V peak-to-peak balanced signal at 110Ω. Inputs are transformer balanced. The Digital Eight or 16 Channel Output Card also conforms to the AES-3 standard. Sample rates of 44.1kHz and 48kHz are set according to settings on the DSP card. There is an option for selecting an external clock source as well. Embedded channel status information is also transmitted with the audio such as channel mode, word length, sample, time of day and block CRC.

The Logic input/output card is a general-purpose interface (GPI) that hosts 12 independent, opto-isolated, solid-state relay inputs and outputs. Customer supplied +5 to +15Vdc is applied to a logic input. The state of that input can be cross connected to a logic output port or can be programmed as an inverted state of the input. Output logic can be programmed in a normally on, off or current state when disconnected from an input port.

The DSP card (digital signal processor) is used by the Bridge to condition and mix audio. In systems configured for radio, two processor cards are installed, one to pre-condition the incoming audio and the second one to perform the mixing bus function. Larger systems use a dedicated card for the master mix.

The Wheatnet Switch is an X-Y router controller for the Bridge.

The number of cards used is dependent on the number of input sources and total mix buses. Frames may be equipped with redundant DSP cards to self heal should a DSP card fail. Mapping DSP cards with their respective mixing surface and frame is accomplished using XP Configuration GUI software.

The CPU card is a full-blown computer in a small package that provides control for the router frame. It is a PC/104 computer with RAM, Flash disk (in place of a hard drive) and an Ethernet port. This device communicates with all the control surfaces, Ethernet controllers and PC XPoint GUI software via TCP/IP. It is backward compatible with legacy Wheatstone products that use RS-485 serial communications.

The Wheatnet Audio Network Card, as alluded to earlier, connects a master chassis to a local chassis via CAT-5e or fiber optic cable. This card also comes in a four-port variety for connecting multiple frames, control surfaces or hubs together. An audio network card transports as many as 64 bi-directional channels of audio. Physical connections include two DB-9 connectors for RS-485, RJ-45 and fiber optic ports for Wheatnet audio connections.

XPoint marks the spot

The XP (XPoint) Configuration GUI is a Windows-based application that configures, monitors and controls the entire router system. Audio source and destination connections are viewable at a glance while sub-menus guide the user to configure hardware, channel definitions and create connection salvos, cross-point locking and system diagnostics.

While the XP Configuration GUI provides extensive configuration and control options on a central PC, the Bridge Digital Audio Network is also controlled by hardware and software interfaces. Rack- and console-mounted XY controllers as well as TCP/IP-based button panels provide tactile input selection.

Three levels of system access to XPoint protect the operation from tampering or accidental changes. The software setup guides the system administrator through Ethernet/IP address changes and various system configuration menus. Each frame or tier must be configured defining what type of card is installed in each slot position.

Most of the sub-menus will be rarely used once the system has been configured. The Cross-point Grid screen will be the most frequently used, perhaps on a daily basis. This 512 × 512 point grid separates the sources vertically from the destination signals horizontally. From here, signal definitions are created denoting the channel name (in eight characters), mode (mono, stereo, 5.1 surround, etc.) and the frame (tier), card and channel where the audio or logic function originates.

Routable logic works similar to its audio counterpart where logic inputs (contact closures or button presses) are cross connected to logic outputs. The duration of the output closure lasts as long as the input state remains in a closed position. These logic signals may be piggy-backed onto audio signals: i.e. when a microphone channel is opened, a logic closure for tally is also active. As many as six logic signals can be ganged together. Triggered port logic is a means by which salvos are activated, such as a talk-back function for temporary audio changes made by contact closures.

Despite the rapid pace to which our world is being reduced to a keyboard, mouse and monitor, there is still a need for push-button panels. Wheatstone has developed a line of TCP/IP-based button panels for this purpose. Once assigned a static IP address, the GP series panel can be programmed to perform specific audio routing and logic changes.

Where more extensive control is needed, short of installing a full-blown control surface, outboard XY Controller modules are used. Each XY controller can access any source or destination signal within the system, or be limited to a specific group of signals. Limiting signal visibility prevents certain users from making unauthorized changes. Signal visibility can be defined on the XY Controller GUI software as well as on stand-alone XY controllers.


Wheatstone has a long-standing reputation in the broadcast industry for durable, reliable and sensibly designed consoles. The Bridge digital audio network system and the companion Generation series consoles are no exception. During my recent installation, I was able to witness first hand the commitment to customer service Wheatstone provided from design all the way through to installation and final configuration. We experienced a couple of software glitches in the beginning that were troublesome, but they were resolved in a timely fashion.

As Wheatstone looks to the future, certain improvements such as expanding the limited eight-character label size might be useful. I found it tedious to locate a specific input or output signal on the XP GUI, especially when you have to browse through 512 entries. It would have been helpful to sort sources alphanumerically as opposed to just a sequence of router channels.

All in all, the Wheatstone Bridge digital audio router system is a well-designed and flexible system, keeping pace with the ever-evolving nature of broadcasting.

Chestnut is assistant chief engineer of the Entercom Kansas City stations.

Editor's note: Field Reports are an exclusive Radio magazine feature for radio broadcasters. Each report is prepared by well-qualified staff at a radio station, production facility or consulting company.

These reports are performed by the industry, for the industry. Manufacturer support is limited to providing loan equipment and to aiding the author if requested.

It is the responsibility of Radio magazine to publish the results of any device tested, positive or negative. No report should be considered an endorsement or disapproval by Radio magazine.

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